You've been able to set an icon for your subscribe page which works as both favicon, brand identifier, and OG image, but — that's a lot of hats to wear. Especially when you consider that icons tend to be, well, square and share images are rectangular.
So we've added a new input for setting a share image. This image will be used as the OG image for your subscribe page, archive pages, and any other pages associated with your newsletter. It will also be used as a fallback for any emails that don't have custom share images set.
I'm working on automatically generated ones for you to use, but that's a little ways off. Check out the GitHub issue for more deets there!